Scavenging of high speed two-stroke internal combustion engines



SCAVENGING OF HIGH SPEED TWO-STROKE INTERNAL GOMB USTION ENGINES Fild Oct. 19'. 19:51 2 Sheets-Sheet 1 /nvento/:

W. BOXAN Feb. 13, 1940.

SCAVENG'ING OF HIGH SPEED TWO-STROKE INTERNAL COMBUSTION ENGINES Filed Oct. 19, 1937. 2 Sheets-Sheet 2 IIIIJYII Inventor:

' Patented F b. 13, 1940 PATENT OFFICE SCAVENGING OF HIGH SPEED TWO-STROKE INTERNAL COMBUSTION ENGINES Walter Boxan, Chemnitz, Germany, assignor to Auto Union Aktiengese n! Applicatlfirli October 19.

lischaft,-Chemnitz, Ger- 1937, Serial No. 169,393

Germany October 26, 1986 4 Claims.

This invention relates to a method of scavenging high speed two-stroke intemalcombustion engines, more particularly those operatingv with a crank casing pump. 4

Port-controlled two-stroke internal combustion engines are already known, in which the scavenging ports lying opposite the exhaust ports are placed at an inclination towards the cylinder cover, for directing the scavengin medium along that part of the cylinder wall which is remote from the exhaust ports towards the cylinder cover and, drivin the exhaust gases in front of them, to. force them out of the cylinder. This fiow along the cylinder walls causes a rotating ll mass to be trapped over the cylinder end, which consists of waste gases and cannot be swept out.

When the equilibrium of this rotating mass is disturbed by the motion of the piston, the flow will adopt another, entirely uncontrolled course no which completely upsets successful scavenging. Such machines depend entirely on the peculiar shape of the scavenging ports, the upper and lower bounding surfaces of the scavenging ports each having a curved form which is a continuous 55 inward curvature in the same direction with respect to the cylinder wall and the radius of ourvature of which continuously increases after .entry into the cylinder. With such an arrangement it is possible only in the case of slow speed In two-stroke engines to obtain stable conditions of flow. In'hig'h speed two-stroke engines this expedient fails, as in this case there canno longer be any stable conditions of flow in the cylinder.

In contradlstinction thereto the novel feature of the invention consists in this, that the scavenging agent is introduced into the cylinder with such pressure and such velocity that-it leaves the scavenging members in the form of free jets m which draw the scavenging medium which has been released by the disintegration oi the jet to themselves and hold it together in the cylinder.

In this way the scavenging agent can be injected into the cylinder in the short period of time available in such a manner'that it will remain in the same. The waste gases are no longer forced out of the cylinder but are only dfluted by the scavenging agent. The scavenging jets entering the cylinder spread according to deflnite laws, somewhat in the shape of a pine-cone, sucking back the scavenging agent which has been released and become mixed with waste gases and keeping it in an ordered circulation in the cylinder. If this circulation be obstructed by a cylinder wall on that side of the scavenging Jet which is remote from the exhaust port and be assisted on the side facing the exhaust port by a depression in the piston, the scavenging jet can be completely deflected towards that side of the cylinder which is remote from the exhaust port. Through the deflection of the scavenging jets a rotating mass is produced in the cylinder, the equilibrium of which is no longer disturbed by the motion of the piston and in which the waste gas can be diluted in accordance with the quantity of scavenging agent injected. In high speed two-stroke engines, in which the flow takes place intermittently, good scavenging is in this way assured, independently of the arrangement and shape of the scavenging members.

The arrangement according to the invention is illustrated as applied to mixture compressing, port-controlled two-stroke internal combustion engines in the accompanying drawings, in which Fig. 1 shows a longitudinal section through the cylinder with unilateral scavenging,

Fig. 2 a longitudinal section through the cylinder in another constructional form,

Fig. 3 a section on line III-III of Fig. 2,

Fig. 4 a longitudinal section through a cylinder in another constructional form,

Fig. 5 a section on line V-V of Fig. 4 and Fig. 6 a longitudinal section through the cylinder with bilateral scavenging.

In the constructional form shown in Fig. 1 the exhaust port 2 and the scavenging port 4 are opposite one another at the inner cylinder end. The exhaust port 2 of rectangular cross-section is cast in the cylinder wall 3 and the scavenging port 4 of round cross-section is bored out of the cylinder wall 5. The scavenging port 4 has the form of a pressure nozzle with a rounded entering edge 8, which is in communication'wlth a relatively wide supply passage '1 for the scevenging agent. The nozzle axis D is inclined at an angle 3 to the cylinder head 6. The supply passage for the scavenging agent is in communication with the crank casing (not shown) through a communicating port which is. controlled by the piston I2.

As it descends, the piston l2 opens first the exhaust port 2 of the vertical height aa, whereby the exhaust gases are expanded. The piston I! then opens the scavenging port 4 of the vertical height as, so that the scavenging agent flows out of the supply passage 1 into the cylinder. In the scavenging port 4 having the form of a pressure nozzle the pressure drop of the scavenging agent is completely converted into velocity, so that the scavenging agent leaves the port as a u exhaust port 2.

free jet 9 and spreads in the cylinder space somewhat in the form of a pine-cone. Through the suction action of the jet 9 at the nozzle opening the spread scavenging agent l0 which mixes with the waste gases is carried back to r the scavenging port 4, so that an ordered circulation of the gas about the jet axis D results, which keeps the scavenging agent l0 together in the cylinder and in this way prevents it leaving prematurely through the exhaust ports 2. Only as the quantity of scavenging agent increases and as the dilution of the waste gases in the cylinder increases will gas ll leave through the Thus, in contradistinction to the so-called short-circuiting bow, a return motion Ila of the gas will be created above the piston l2 towards the nozzle opening as the place of greatest vacuum in the cylinder.

In the constructional example shown in Figs. 2 and 3 the scavenging port 4 has the form of a pressure nozzle of rectangular cross-section, which is directed towards the cylinder head 6 and the upper edge of which merges with a curve l3. into the cylinder wall 5. The curvature I3 is formed in the manner of an aero-ioil profile l4. The upper edge of the scavenging port 4 is formed by an insertion piece It of the cylinder wall 5, which is fixed by screw bolts It to the cylinder. The piston l2 has a depression which increases in depth towards the side I! of the scavenging port 4. 0n the piston l2 descending, the scavenging port 4 will first open to the extent 0'6,- allowing burnt gas partially to expand in the supply passage 1. 'I'hereupon the exhaust port 2 opens with the vertical height ca, allowing the cylinder to discharge. Only after the completion of the exhaust the communicating port (not shown) at the bottom end oi the supply passage 'I opens, so that the scavenging agent can emerge in the form of a jet through the scavenging port 4 which at this moment will already have opened sufliciently far. As the admission of the gases II to the scavenging jet 9 is obstructed at the sideremote from the exhaust port 2 by the screening action of the cylinder wall 5 and is assisted on the side facing the exhaust port 2 by the suction action of the piston depression l1, the scavenging jet 9 will be deflected through a certain angle '7. Owing to the curvature l3, the deflection of the scavenging jet 9 takes place so gradually that the scavenging jet 9 will adhere to the cylinder wall 5. The scavenging agent In which becomes liberated by the disintegration of the jet and mixed with the residual gases is thus only drawn back against one side of the scavenging jet 9, causing it to force the scavenging jet 9 against the cylinder.

wall 5. The gases ll leaving through the exhaust port 2 are thus only relatively slightly diluted by the scavenging agent.

In the example shown in Figs. 4 and 5 better scavenging conditions are obtained through the scavenging port 4 being rounded out not only at the upper and lower edge at 8, l3, but also at the lateral edges at l8. The scavenging port 4 and the exhaust port 2 are each provided with a central bridging piece for the better guiding of the piston rings. The scavenging agent thus enters the cylinder in the form of two jets 9 which are also deflected laterally against the cylinder wall 5, so that the jet takes the form of a relatively thin layer 90., along which the scavenging medium In which has been liberated through the disintegration oi the jet is carried,

both in the longitudinal plane (Fig. 4) and in the transverse plane (Fig. 5). In this case the piston depression I! is made central.

In the constructional example shown in Fig. 6 the method is applied for bilateral scavenging. Two exhaust ports 2, 2' and two scavenging ports 4, 4' are in this case disposed respectively cross-wise at the inner cylinder end. In this case the flow ill of the scavenging agent is symmetricalwith respect to a longitudinal plane of the cylinder, which is determined vby the exhaust ports 2, 2'. As in this case smaller nozzle cross-sections will suflice, both 0. more favourable heat stressing of the cylinder and better scavenging will result.

The nearer the pressure drop of the scavenging agent between the supply passage I and the cylinder approaches the critical pressure drop, the more compact will the scavenging jet be, where it emerges, and the better will consequently be the eflect of the scavenging. An increase of the pressure drop above the critical drop must however be avoided, as in this case the jet would emerge at a greater pressure than the cylinder pressure and would therefore not retain its form in the cylinder. When the pressure drop approaches the critical drop, the velocity of the jet will approach the velocity of sound, which is the correct one for the state of the scavenging agent. The quantity of jet introduced per time unit is calculated not from the smallest nozle cross-section ad, but in view of the constriction of the jet from the smallest jet cross-section as. The angle of deflection 'y of the scavenging jet depends in the first place on the angle of inclination p of the scavenging port 4 with respect to the cylinder axis. With a complete deflection of the scavenging jet 9 against the cylinder wall 5 (Figs. 2 and 4) the angle of deflection '7 together with the jet angle a form the angle of inclination e.

The method may be applied to all forms of jet, irrespective of whether transverse, reverse or quick flow oi scavenging is employed andirrespective of whether arrangements with a single port or several ports are provided. The method is particularly suitable for high'speed two-stroke engines having crank casing pumps, as in such engines, owing to the relatively high scavenging pressure, great scavenging velocities occur, which favour the forming and deflection of the scavenging jets.

What I claim is:

l. A method of charging a high-speed twostroke internal combustion engine having a cylinder, an exhaust port, an inlet nozzle and a piston controlling fluid passage through the port and nozzle consisting in passing a charge through the nozzle at such pressure and velocity as to cause the charge to enter the cylinder as a free jet and thereby exert a suction eilect upon the part of the charge released by the dispersion of the jet to produce an eddy tending to collect and hold the charge in the cylinder.

2. In a two-stroke internal combustion engine the combination of a cylinder, an exhaust port, an inlet nozzle and .a piston controlling fluid passage through the port and nozzle, said nozzle being adapted to direct a charge into the cylinder at such pressure and velocity as to cause said charge to enter the cylinder as a free jet and thereby exert a suction efiect upon the part of the charge released by the dispersion of the jet to produce an eddy tending to hold the charge in the cylinder, with said piston being hollowed in the p01 tion thereof facing said nozzle.

3. A two-stroke internal combustion engine including a cylinder, an exhaust port, an inlet nozzle and a piston controlling fluid passage through the port and nozzle, said nozzle being adapted to direct a charge into the cylinder at such pressure and velocity as to cause said charge to enter the cylinder as a tree Jet and thereby exert a suction effect upon the part of the charge released by the dispersion of the jet to produce an eddy tending to hold the charge in the cylinder, the wall of the cylinder adjoining said nozzle being rounded to merge with said nozzle, said nozzle having an aerotoil profile.

4, A two-stroke internal combustion engine including a cylinder. an exhaust port, an inlet nozzle and a piston controlling fluid passage through the port and nozzle, said nozzle bein adapted to direct a charge into the cylinder at such pressure and velocity as to cause said charge to enter the cylinder as a free Jet and thereby exert a suction effect upon the part or the charge 7 released by the dispersion of the jet to produce an eddy tending to hold the charge in the cylinder, a member inserted in the cylinder wall and presenting a rounded surface joining the nozzle and cylinder wall.

WALTER BOXAN. 

